A fuel cell stack is an energy conversion system for supplying a fuel gas and an oxidizing gas to a membrane-electrode assembly to cause an electrochemical reaction, thereby converting chemical energy into electric energy. Above all, a solid polymer electrolyte type fuel cell stack in which a solid polymer film is used as an electrolyte is easily made compact at a low cost, and additionally it has a high output density, so that the use application of the stack as a car-mounted power source is expected.
In a gas passage of a fuel cell, formed water generated by the electrochemical reaction of a reactant gas, humidification water for humidifying the reactant gas and the like remain. If power generation is stopped while leaving this remaining water, the remaining water freezes in an environment at a low temperature, the diffusion of the reactant gas to the membrane-electrode assembly is prevented, and low-temperature starting properties lower. In view of such a problem, in Japanese Patent Application Laid-Open No. 2002-246053, a technique is suggested which supplies a scavenging gas into the fuel cell stack at an operation stop remove a water content and which measures an AC impedance of the fuel cell stack to judge the dry degree of an electrolytic film.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-246053